The present invention relates generally to light sensors, and more particularly, to a light sensor and thresholding method for use with a toll road revenue collection system that minimizes transmission of redundant data between the light sensor and a processor that controls the contrast level of images produced by a license plate camera.
The assignee of the present invention has developed an open road toll road revenue collection system wherein vehicles do not have to stop at a toll booth or collection station to pay toll fees. Each vehicle may be equipped with a transponder that transmits identification data to the system that is used to determine the time of entry into and exit from the toll road and to bill the owner of the transponder. However, casual users having vehicles that do not have a transponder may also use the toll road. In these cases, the toll collection system uses one or more license plate cameras to image the vehicle license plate, and the image is processed to determine the owner of the vehicle that is to be billed. The light sensor is used in capturing a readable image of the license plate.
Therefore, in cases of non-transponder equipped vehicles, the vehicle license plate must be accurately imaged, recorded, and processed to ensure proper billing. This must be done in all types of weather conditions, and in particular, during constantly changing sun conditions. If a cloud moves in front of the sun, it quickly changes the power level incident on the vehicle license plate, and this requires that the shutter, gain, and pedestal settings of the cameras be rapidly changed to properly image the vehicle license plate. The pedestal setting of the camera refers to black level definition.
Furthermore, vehicles entering and exiting the toll road may travel very quickly, and the amount of time available to image the license plate is small. Consequently, the shutter, gain, and pedestal settings of the license plate camera, and hence the contrast level between the background and the vehicle license plate, must be changed quickly to ensure the best image quality. Such contrast changes occur randomly, and are a function of local weather conditions.
To control the contrast level of the image viewed by the license plate camera, a light sensor is used to monitor the power output from the sun which provides message reports to a processor that controls the shutter, gain, and pedestal settings of the license plate camera. The light sensor is designed to look at a replica of a license plate, which reflects the sun's power into the light sensor. The light sensor is aimed in the same direction as the license plate camera, and is designed so that it senses the same relative light power as the camera. Therefore, changes in the sun's power level reflected from the replica license plate sensed by the light sensor are used to control the shutter, gain, and pedestal settings of the license plate camera so that the actual vehicle license plates are properly imaged thereby.
A previously developed light sensor used for this purpose was designed to transmit a message report every one-tenth of a second. This results in the transmission of 36,000 message reports per hour, which are sent to the host processor for processing. When using the previously developed light sensor, data was continuously transmitted between the light sensor and the processor, even at night. This volume of message reports required the processor to decode and process redundant data. It was therefore determined that it would be advantageous to reduce the overall processing requirements of the processor resulting from data transmitted by the light sensor.
Accordingly, it is an objective of the present invention to provide for a light sensor and thresholding method that minimizes transmission of redundant data between a light sensor and a processor that controls the shutter, gain, and pedestal settings of a license plate camera and thus reduces the processing requirements of the processor. It is a further objective of the present invention to provide for a light sensor that reduces the processing requirements of processors used in a toll road revenue collection system.